Figure 7.1A DSC and DDSC (broken line) traces for a polymer sample (source: Netzsch GmbH)
The figure displays an endothermic shoulder at around 80 °C, an exothermic peak maximum at 150 °C and an endothermic peak maximum at 250 °C.
Temperature-modulated DSC (TMDSC) is a technique (Reading 1993), which is devised to declutter the information gleaned from a DSC run and make it easier to identify and measure properties that are not so distinctly evident on a normal DCS trace, e.g., glass transitions for semi-crystalline polymers. In TMDSC, the sample is subjected to an oscillating heating or cooling rate "forcing function", where the mean temperature changes linearly (e.g., a sinusoidal modulation is superimposed on the underlying heating ramp). The process enables greater sensitivity and separates overlapping thermodynamic/reversing transitions (glass transition, melting) from non-reversing/kinetic events such as crystallisation, curing and decomposition. The reversing signal is associated with properties dependent upon the temperature rate of change while the nonreversing signal defines kinetic events (i.e., those associated with both time and temperature).
Other ways of affecting sensitivity/resolution include Fast Scan DSC or Hyper DSC techniques that apply very high heating rates (i.e., fast scans) to a sample to increase the sensitivity of a DSC to transitions (particularly useful with otherwise hard to detect small transitions) and to jump kinetic behaviour. As a DSC experiment is accelerated in the Hyper-DSC method, the same heat flow occurs over a much shorter period of time and provides the heat required for the event almost instantly and, therefore, the thermal event becomes magnified. This allows extremely low-energy transitions to be identified and measured with ease. The technique obscures sample changes, such as crystallisation, curing and decomposition that require time and, therefore, can occur during slow heating when using conventional DSC. During a very fast scan, the sample does not have the time to undergo any structural changes and, therefore, it is possible to maintain the material analysed in its "as-received" state.
The Hyper-DSC method is only possible with power-compensated DSC, since the single furnace DSCs cannot heat at the required rates. Fast scan heating rates range from 100-300 °C/min, whereas Hyper-DSC heating rates range from 300-750 °C/min. When heating rates of 100-750 °C/min are applied, the response of the DSC to weak transitions is enhanced and it is possible to detect very low levels of amorphous materials: the limit of detection with conventional DSC is about 10% amorphous, and with Hyper-DSC this becomes approximatelyl%. High scan rates also make it is possible to conduct faster tests and run many more samples.
Introduction to Polymer Science and Technology
Thermal properties that can be measured with DSC were mentioned above, and most of which are simply extracted from a DCS graph, e.g., T , Tc and Tm, see Figure 7.2, and others require calculations as described in the succeeding sections.
1 / \
transition/ ÀÑÐ _____________ / _Jd_
exo iT 1 'ä
Figure 7.2Illustration of a DSC scan for a semicrystalline polymer sample
7.1.1 Degree of crystallinity
The degree of crystallinity of a material can be calculated using the appropriate heat of fusion values:
Introduction to Polymer Science and Technology Thermal properties
% crystallinity = (ÄÍò /AHg,) x 100
where, AHm is the area under the melting peak when heat flow is plotted against time from a DSC run, see Figure 7.2, and ^Hm is the literature value for the same material in 100% crystalline state, Table 7.1 shows the AHJ^ values for some semi-crystalline polymers. The method is standardised in ISO 11357-3:2011. An example calculation of % crystallinity for a sample of PEEK is shown in Section 4.3.4.
AHm is determined as shown in the following text box, when the DSC trace is a plot of "heat flow vs. temperature".
Note that AHm = heat flow x time = (------------ ) x time
Time x mass
"Time" could be expressed in terms of temperature from the rate of heating (°C/min) used in the DSC scan, time = (temperature) / (heating rate), and if substituted in the above equation, it gives
.TTf heat ë ^temperature . J ê ë
AHm = (-------------- ) x (—---------- ); its unit being (——) x (—) or (J/g).